91Ó°ÊÓ

The primary standard is a chemical substance that is used as a reference to determine the unknown concentration of a substance in a solution or any mixture. The primary standard

• must be available with high purity

• must be stable under the conditions of proper storage.

• should have a higher molecular weight so as to minimize the effects caused by errors in weighing.

Tris(hydroxymethyl)aminomethane can be used to standardize$\mathrm{HCI}$. Its formula is ${\mathrm{H}}_2\mathrm{N}-\mathrm{C}{\left({\mathrm{CH}}_2\mathrm{OH}\right)}_3$.

Some other primary standards that can be used are:

Mercuric oxidewith the formula HgO.

Sodium carbonate with the formula ${\mathrm{Na}}_2{\mathrm{CO}}_3$.

Borax with the formula ${\mathrm{NaB}}_4{\mathrm{O}}_{7-}10{\mathrm{H}}_2\mathrm{O}$.

Potassium hydrogen phthalate is used to standardize $\mathrm{NaOH}$and its formula is ${\mathrm{C}}_6{\mathrm{H}}_4{\left(\mathrm{COOK}\right)}_2$.

Some other primary standards that can be used are:

Potassium hydrogen iodate with the formula${\mathrm{KHIO}}_3$

Oxalic acid dihydrate with the formula ${\left(\mathrm{COOK}\right)}_{2}2{\mathrm{H}}_2\mathrm{O}$.

Sulfamic acid with the formula ${\mathrm{H}}_2\mathrm{N}-{\mathrm{SO}}_3\mathrm{H}$.